1. Field of the Invention
The present invention relates to a process for producing biaxially oriented polymer sheet and an apparatus for performing such process. More particularly, the invention is directed to a process wherein a polymer slab is introduced into a deformation zone and is advanced through the zone while the working face of at least one working tool in the zone is ultrasonically excited. As used herein, the term "polymer" includes polymers selected from the group consisting of polymethyl methacrylate, polyvinyl chloride, polyethylene, polystyrene, polycarbonate, polyamides, polyester such as polyethylene terephthalate, polyarylates, nylons and mixtures thereof.
2. Description of the Art
Biaxially oriented polymers have numerous advantages over unoriented polymers in that they possess higher strength, improved stiffness, increased toughness, and superior thermoformability. Biaxially oriented polymers have been successfully produced by a number of techniques. These techniques include hydrostatic extrusion, such as that disclosed in U.S. Pat. No. 4,282,277, which is a process of using a mandrel in combination with a conical die to form a tubular product. The tubular product is then cut, flattened and annealed.
In order to avoid the expense of first forming a tube and subsequently slitting and flattening the tube to form a biaxially oriented sheet, a twin-belt process has been developed such as that disclosed in U.S. Pat. No. 4,789,514. Such twin-belt process provides substantially uniform strain over the width of the sheet and through the thickness of the sheet. The material of the workpiece, proximate the center portion, is squeezed in a manner to compensate for the friction between the material and the working surface. It is also known that lower pressures are required to produce biaxial orientation by a twin-belt process as compared to a process using stationary flat dies. In actual experiments, pressures of 7,000 psi (492 kg/cm.sup.2) were required to form biaxially oriented sheet between flat dies, whereas pressures of only 800 psi (56.2 kg/cm.sup.2) were required to form biaxially oriented sheet in a twin-belt machine. Because the flat die machines typically require higher extrusion pressures, they require greater initial capital outlays than twin-belt apparatus.
The use of ultrasonics has been disclosed in the metalworking art. For example, U.S. Pat. No. 3,318,129 discloses a metal rolling process in which ultrasonic vibration is applied to the metal being rolled through the roller to eliminate the need for annealing. U.S. Pat. No. 2,408,627 discloses the use of a vibration generating device attached to an extrusion die. U.S. Pat. No. 4,793,954 discloses the use of ultrasonic vibration in molten thermoplastic shearing operations such as extrusion and injection molding.
Despite significant progress in the area of producing biaxially oriented polymer sheet, there is a need and a demand for further improvement. For instance, the pressure required to process highly frictional materials, such as polyethylene terephthalate, by axial deformation exceeds the pressure limitations of conventional state-of-the-art twin belt processing machinery, which limit is equivalent to about 1500 psi. As will be explained in detail below, the process of the present invention utilizes ultrasonic processing which has the effect of reducing friction to thereby allow the processing of materials, including highly frictional materials, by biaxial deformation.
Accordingly, a new method and apparatus for producing biaxially oriented polymer sheet are desired which combine the use of ultrasonics to further improve the process of producing biaxially oriented polymer sheet.